Field of the Invention
This invention patent relates to a closure for food containers. More particularly, this invention relates to a metal closure having an oxygen scavenging interior surface for use in sealing a food container such as a transparent plastic can.
Description of the Related Art
With increases in the price of metal, plastic cans have become price competitive with two piece metal cans. Other benefits of plastic cans include the potential to view the can contents through the can body, the elimination of bisphenol A (BPA) (a component in the epoxy resin used to coat the insides of many food and beverage metal cans), and positive consumer perceptions regarding freshness and taste.
High Barrier Plastic Cans
High barrier, multilayer plastic cans fall into two basic types: those having tubular extruded bodies requiring two closures or ends, and those having thermoformed or blow molded bodies requiring a single end. Either type can suffer from two negative issues: headspace oxygen and retort shock.
Headspace Oxygen
“Headspace” refers to the space in a can or container not occupied by product. Generally due to handling issues, the headspace in a typical can may be as high as 0.5% of the can volume. The air in the headspace or, more specifically, the oxygen in the air, can reduce product shelf life. Solutions to this problem include flushing the headspace with an inert gas such as nitrogen and using a chemical absorbent to absorb the oxygen.
Retort Shock
Ethylene-vinyl alcohol copolymer (EVOH) is the most common oxygen barrier plastic material used in the production of plastic barrier containers. Unfortunately, the oxygen barrier property of EVOH decreases as the barrier layer absorbs water, which occurs during hot retort processing. Oxygen from the ambient air can pass through the barrier layer during the drying stage, a phenomenon referred to as “retort shock.”
Possible solutions to retort shock include using a desiccant (drying agent) in the can body, and/or using an oxygen scavenger in the can body.
Oxygen Scavengers
Oxygen scavengers come in two types: those that do not require an activator (and so begin working as soon as the bottle or can manufacturing process is finished) and those that require an activator (such as ultraviolet (UV) light or moisture). Oxygen scavengers requiring UV light activation are not popular because they require an extra processing step. Oxygen scavengers activated by moisture are generally regarded as safe (GRAS) for use in food containers, but sometimes can cause opaqueness, an undesirable property when a clear or transparent container is desired.
The Prior Art
Various references describe ways to address the problems of headspace oxygen, retort shock or both. The majority of these references disclose the use of oxygen scavengers in a plastic container body wall. However, it has been discovered that adding an oxygen scavenger solely to the interior side of the metal end of a plastic container can effectively address the problem of headspace oxygen and possibly also retort shock.
It is therefore an object of the present invention to provide a way to use an oxygen scavenger in a plastic container without compromising the clarity of the plastic container body.
Another object of the present invention is to provide a metal end or closure having an oxygen scavenging interior surface for use with a clear or transparent plastic container body.
Further and additional objects will appear from the description, accompanying drawings, and appended claims.